There are a huge amount of pressed coals under buildings, railroads, and water-bodies in China, which are added up to 13.79 billion tons, according to the incomplete statistics made by major state-owned coal mines in China.
As the technical level of coal mining is improved, techniques for mining pressed coal under buildings, railroads, and water-bodies have been developed greatly. Presently, commonly used surface subsidence controlling methods mainly include: coordinated mining, partial mining, grouting into separated strata in overburden, and mining with filling, etc. Coordinated mining utilizes the counter balancing among surface deformations resulted from simultaneous mining at multiple working faces to attain the purpose of reducing surface deformation, but it involves simultaneous mining at multiple working faces, and is constrained by the layout of protected objects on the ground surface; therefore, it has great impacts on the pioneering design and has limited applicability. Partial mining controls overlying strata movement and surface subsidence by mining coal partially and reserving permanent coal pillars in certain width, and mainly includes wangeviry mining, room and pillar mining, strip mining, limited thickness mining, knife mining, and roadway mining, etc. Though these methods can control surface subsidence to certain degree, their recovery ratios are not high, usually about 50%. Grouting into separated strata in overburden utilizes bore grouting to fill the space of separated strata between upper hard formation and lower soft formation, to attain the purpose of controlling the overlying bed subsidence above the hard formation and the surface subsidence; with that method, the surface subsidence reduction ratio is usually not higher than 40%. Compared with other surface subsidence controlling methods, mining with filling is the most effective method for controlling surface subsidence at present, and the goaf can be filled partially or fully. Depending on the layout of working face, the main method can be goaf filling along long-wall working face, strip filling, or roadway filling, etc. The problems existing in these methods mainly include: difficulties in coordination between coal mining and filling, complex filling system, large filling space, and long filling time, etc.